This invention relates to a method for the determination of hexavalent chromium (CrVI). More specifically, the present invention relates to a simple, fast, sensitive, and economical method for the determination of CrVI which is especially adapted for environmental and work place samples (including solid and air samples). The present method can be used in both laboratory and field analysis.
Chromium exists primarily in two valence states, trivalent (CrIII) and hexavalent (CrVI). The trivalent state is relatively non-toxic, and is an essential nutrient in the human diet. On the other hand, CrVI has been shown to be a human respiratory carcinogen in epidemiological studies of workplace exposures, and has been classified by the U.S. Environmental Protection Agency (EPA) as a Group A inhalation carcinogen. Hence, analytical methods are desired which can be used to easily speciate chromium so that human exposures to CrVI can be monitored and, thus, better controlled.
Workplace exposure to CrVI has been associated with a number of industrial sources, such as metal plating, spray painting, welding, tanning, and abrasive blasting operations. Environmental sources of CrVI include, for example, deteriorated or disturbed chromate-containing paint, combustion sources such as automobiles and incinerators, and fugitive dusts from contaminated solid. Because of the desire to accurately measure CrVI at low levels, the development of analytical methods for the determination of CrVI has been a subject of significant interest in occupational and environmental health.
Chromium has been detected using atomic absorption spectrometry (Mehra et al., Talanta, 1989, 36(9), 889; Fong et al., Spectrosc. Lett. 1991, 24, 931), atomic absorption spectrometry (Jarvis et al., Analyst, 1987, 122, 19; Arar et al., Environ. Sci. Technol., 1992, 29, 1944); atomic emission spectrometry (Boumans, Line Coincidence Tables for Inductively Coupled Plasma Atomic Emission Spectrometry, Oxford University Press, Oxford, 2nd ed., 1984; Giglio et al., Anal. Chim. Acta, 1991, 254, 109; Roychowdhury et al., Anal. Chem., 1990, 62, 484), X-ray fluorescence (Arber et al., Analyst, 1988, 113, 779), charged-particle X-ray emission spectrometry and neutron activation analysis. National Institute for Occupational Safety and Health (NIOSH) Methods 7024 and 7300 (NIOSH Manual of Analytical Methods, Eller and Cassinelli (eds)., National Institute for Occupational Safety and Health, Cincinnati, Ohio, 4th ed, 1994) use atomic absorption spectrometry and atomic absorption spectrometry, respectively, for the determination of chromium in workplace air samples. These methods, however, generally determine only total. Moreover, these methods generally involve expensive and complex instrumentation and are not, therefore, generally suitable for monitoring directly in the field.
Spectrophotometric and colorimetric methods have been developed for the determination of CrVI. See, for example, Alvarez et al., Talanta, 1989, 36(9), 919; Haukka, analyst, 1991, 116, 1055; Abel et al., Am. Ind. Hyg. Assoc. J., 1974, 35, 229. The most prevalent colorimetric method uses the selective reaction of CrVI with 1,5-diphenylcarbazide (DPC) under acidic conditions to yield a red-violet CrVI-diphenylcarbazone complex. A variation of this colorimetric method is used in NIOSH method 7600 (NIOSH Manual of Analytical Methods, Eller and Cassinelli (eds.), National Institute for Occupational Safety and Health, Cincinnati, Ohio, 4th ed, 1994) where alkaline extraction is used to help stabilize the CrVI species. Stripping voltammetry (Wang et al., Analyst, 1992, 117, 1913; Elleouet et al., Anal. Chim. Acta, 1992, 257, 301) and ion chromatographic assays (Powell et al., Anal. Chem., 1995, 67, 2474; Vercoutere et al., Mikrochim. Acta., 1996, 123, 109; Molina et al., Am. Ind. Hyg. Associ. J., 1987, 48, 830; ASTM D 5281-92, xe2x80x9cStandard Test Method for Collection and Analysis of Hexavalent Chromium,xe2x80x9d in Annual Books of ASTM Standards, American Society for Testing and Materials, vol. 11.01, Philadelphia, Pa., 1992; U.S. Environmental Agency, Method 218.6. Determination of Dissolved Hexavalent Chromium in Drinking Water, Groundwater and Industrial Waste Water Effects by Ion Chromatography, EPA Office of Research and Development, Cincinnati, Ohio, 1990; U.S. Environmental Agency, Method 3060A, xe2x80x9cAlkaline Digestion for Hexavalent Chromiumxe2x80x9d in Test Methods for Evaluating Solid Wastes, EPA, Washington, D.C., 1995) have also been used to determine CrVI in various samples. Many of these techniques are limited to laboratory-based analysis and cannot, therefore, be used in field monitoring and/or real-time evaluations.
Over the past decade, solid phase extraction (SPE) has been established in the analytical chemistry laboratory and has become increasingly popular. The use of SPE for the separation and preconcentration of trace polar or non-polar target analytes has been widely investigated, and the advantages of such a technique over a conventional liquid-liquid extraction, coprecipitation, electrochemical deposition and evaporation have been well documented. See, for example, Masque et al., Analyst, 1997, 122, 425-428; Corcla et al., Environ. Sci. Technol., 1994, 28, 850-858; Shahteri et al., J. Chromatogr., 1995, 697, 131-136. Some of the advantages of SPE over classical analytical methods include: (1) efficiency and simplicity; (2) solvent minimization and enhanced safety with respect to hazardous samples; (3) high preconcentration factors; (4) good recoveries; (5) flexibility; and (6) low cost.
Both off-line and on-line SPE methodologies have been employed for the preseparation and preconcentration of a variety of analytes. Off-line SPE methodologies involve the use of packing materials that may contain functional groups of different polarity such as C8 or C18 bonded silica phases (Falco et al., Analyst, 1997, 122, 673-677). With on-line SPE, followed by high pressure liquid chromatography, a critical parameter is the selection of an adequate precolumn in order to avoid band broadening of the first eluded peaks, and to allow for the percolation of large sample volumes (Kiss et al., J. Chromatogr., 1996, 725, 261-272). Numerous solid-phase extractants, such as pure or modified silica, alumina, magnesia, activated carbon, polyurethane, and cellulose and its derivatives, have been used in SPD techniques. Consequently, solid phase extraction has largely replaced classical liquid-liquid extraction in the analytical laboratory.
More recently, innovative new cartridges for SPE, such as reversed-phase and ion-exchange of target analytes in a single resin, are being developed and many are commercially available. These resins cartridges allow for sorption of the analyte of interest while removing non-sorbed interferents, with fast, quantitative adsorption and high elution capacities. These cartridges can improve the overall specificity and sensitivity of trace analysis. Furthermore, the use of commercially available, low cost vacuum manifolds for SPE allows for up to, and even greater than, 24 samples to be processed simultaneously. Complete automation of procedures based on SPE are also available with commercial instrumentation. Despite these advantages, there have been relatively few applications of SPE to inorganic materials, including heavy metals.
Ultrasonic extraction (UE) for the purpose of dissolving target heavy metal analytes in environmental samples is also a technique that has not been used extensively, although it offers great promise (Lugue de Castro et al., Trends Anal. Chem., 1997, 16, 16-24). UE has been demonstrated to perform well for the quantitative dissolution of several heavy metals in a variety of environmental matrices (Harper et al., Anal. Chem., 1983, 55, 1553-1557; Sanchez et al., Analysis, 1994, 22, 222-225; Ashley, Electroanalysis, 1995, 7, 1189-1192z), including hexavalent chromium (James et al., Environ. Sci. Technol., 1995, 29, 2377-2381).
Nonetheless, there remains a need for a simple, reliable, fast, inexpensive, and field-based method for the detection of CrVI in environmental and workplace samples. This invention, combining the use of ultrasonic extraction and strong anion exchange solid phase extraction, provides such a method. This method provides a novel and effective approach for the on-side determination of CrVI in environmental and workplace samples.
This invention relates to a method for the determination of hexavalent chromium (CrVI). Based on the chemical properties of chromium species in aqueous solutions, a simple, fast, sensitive, and economical field method has been developed and evaluated for the determination of hexavalent chromium (CrVI) in environmental and workplace air samples. By means of ultrasonic extraction in combination with a strong anion exchange solid phase extraction (SAE-SPE) technique, the filtration, preconcentration, and isolation of CrVI in the presence of other chromium species and interferents was achieved. The method generally involves: (1) ultrasonic in basic buffer solution to extract CrVI from environmental matrices; (2) strong anion exchange solid phase extraction to separate CrVI from other chromium species and potential interferents; (3) acidification of the eluate containing the CrVI ions; (4) complexation of CrVI with a complexing agent to form a soluble, colored CrVI-complex; and (5) spectrophotometric determination of the colored CrVI-complex. Preferably, the ultrasonication step is carried out in the presence of a slightly basic ammonium buffer and the complexing agent is 1,5-diphenylcarbazide. This present method can effect the extraction of both soluble (K2CrO4) and insoluble (K2CrO4) forms of CrVI without inducing CrIII (Cr2O3) oxidation of CrVI reduction. The method allows for the dissolution and purification of CrVI from environmental and workplace air sample matrices for up to 24 samples (or even higher numbers) simultaneously in less than about 20 minutes (excluding the ultrasonic extraction time). The present method is simple, fast, quantitative, and sufficiently sensitive for the determination of occupational exposures of CrVI. The method is especially applicable for on-site monitoring of CrVI in environmental and industrial hygiene samples, including both solid samples (e.g., soil, paint chips, dust, solid residues, and the like) and air samples.
One objective of the present invention is to provide a method for the detection of CrVI in a sample, said method comprising:
(1) ultrasonic extraction of CrVI from the sample utilizing a first buffer solution (i.e., the ultrasonic buffer) having a slightly basic pH, whereby the pH of the first buffer solution is such that neither significant CrIII oxidation nor CrVI reduction occurs;
(2) separation of the CrVI in the ultrasonic extractant from step (1) from any CrIII or other interferents that might be present in the sample by passage of the ultrasonic extractant through a strong anion exchange solid phase extraction media;
(3) elution of the CrVI from the media with a second buffer solution (i.e., the elution buffer) having a slightly basic pH;
(4) acidification of the eluate containing the CrVI; and
(5) addition of a complexing agent to the acidified eluate to form a colored CrIV-complex if CrVI is present in the sample. The ultrasonication step employed to liberate CrVI from the sample matrix is preferably carried out using a slightly basic ammonium buffer and the complexing agent is preferably, 1,5-diphenylcarbazide. Preferably, the amount of CrVI present in the sample is determined using any appropriate technique. More preferably, the amount of CrVI present in the sample is determined using a simple and direct spectrophotometric procedure which can be used in the field or on-site.
Another objective of the present invention is to provide a method for the quantitative detection of CrVI in a sample suspected of containing CrVI, said method comprising:
(1) ultrasonic extraction of CrVI from the sample utilizing a first ammonium buffer solution (i.e., the ultrasonic buffer) having a slightly basic pH, whereby the pH of the ammonium buffer solution is such that neither significant CrIII oxidation nor CrVI reduction occurs;
(2) separation of the CrVI in the ultrasonic extractant from step (1) from any CrIII or other interferents that might be present in the sample by passage of the ultrasonic extractant through a strong anion exchange solid phase extraction media;
(3) elution of the CrVI from the media with a second ammonium buffer solution (i.e., the elution buffer);
(4) acidification of the eluate containing CrVI;
(5) addition of 1,5-diphenylcarbazide to the acidified eluate to form a colored CrVI-1,5-diphenylcarbazone complex if CrVI is present in the sample; and
(6) subjecting the CrVI-1,5-diphenylcarbazone complex, if present, from step (5) to a spectrophotometric analysis in order to determine the amount of CrVI present in the sample.
These and other objectives and advantages of the present invention will be apparent to those of ordinary skill in the art upon consideration of the present specification.